A polyester film, particularly biaxial oriented polyethylene terephthalate film is widely used for base film for photosensitive materials, base film for metal vacuum evaporation, film for wrapping materials, base film for magnetic recording materials, film for graphic art and so on in wide variety of fields because of excellent transparancy, dimensional stability, heat-resistance, chemical resistance and electrical properties. In the each use of film, the coating materials, for example, the paints including a sensitizer, matting agent, magnetic material and the like are coated on the biaxial oriented polyester film. However, in spite of excellent physical and chemical properties, biaxial oriented polyester film are lower affinity and adhesion with the coating materials.
Thus, there has been proposed various means for increasing the adhesion and the affinity of the film surface with the coating materials. For example, there have been proposed a method in which the film surface being reacted with the coating materials is coated with the suitable primer layers in the last step, a method in which the film surface is treated with various chemical agents and a method in which the film surface is activated by corona discharge, plasma and IR irradiation.
However, the first method has drawbacks that the adhesion of the base film and polyester films or various coating materials is lowered and it is undesirable economically and technically that a coating process of primer layer is instituted.
Also, the second method provides good adhesion of the film surface and various coating materials, whereas it has drawbacks that handling and treating of the chemicals are difficult and contamination of the environment is raised.
In addition, the last method has a good initial adhesion with the coating materials, whereas it has a drawback that the adhesion is decreased by the passage of time.
Further, there has been known the compositions for primer layer using the water as a solvent. Particularly, it has been proposed the water-solution or water-dispersed solution of polyester copolymer as follows;
(1) Japanese Publication No. Sho 47-40873 describes a method in which polyester is dispersed to water comprising of more than 8 mol % of the compounds containing sulfonic acid metallic base to the total acid and more than 20 mol % of polyethylene glycol to the total glycol.
(2) Japanese Publication No. Sho 56-5476 describes a method in which combination of 3.5.about.7.5 mol % of the sulfonic acid alkali metallic base being able to form ester as a polyester copolymerization component and 30.about.100 mol % of diethylene glycol as a glycol component is used.
(3) Japanese Laid-open No. Sho 56-88454 describes a mothod in which combination of resins containing 0.5.about.10 mol % of aromatic dicarbonic acid containing sulfonic acid metallic base as a polyester copolymerization component and water containing 2.about.66% of aqueous organic compounds having 60.degree..about.200.degree. C. of boiling point as the dispersion solution is used.
However, the method of (1) has a defect of lowering a water-resistance. And, in the method of (2), when the sulfonic acid metallic base being able to form ester is added unsufficiently, although a large quantity of diethylene glycol are used to increase a water-resistance, it is unsatisfied.
As well, a water-resistance is increased to some extent in the method of (3), but this method has a defect unsutable to in-line treatment process as the organic solvent must be used.
In the mean time, the prior methods for improving the friction coefficient of the polyester film include the precipitation induced particle method and particle addition method.
However, these methods are effective to decrease a friction coefficient, but has a defect of exsiting fine particles in the film composition and thereby the problems of lowering a transparancy and producing a voids in film are generated. Particular, the above problems are to be an important obstructions in microfilms and films for photography which are required an excellent transparancy. When a friction coefficient is high, an operation is worse, and in case that a transparancy is low, it is difficult to obtain a clear picture.